Ethylene glycol is a valuable commodity chemical that has a broad range of uses as both a building block for other materials such as polyethylene terephthalate (PET) and for its intrinsic properties such as for antifreeze. Ethylene glycol demand is substantial, making it one of the largest volume organic chemicals produced in the world. It is currently made by multistep processes which start with ethylene derived from hydrocarbon feedstocks.
Proposals have been made to manufacture ethylene glycol from renewable resources such as carbohydrates. See, for instance, U.S. Pat. No. 5,210,335; EP 2419393; U.S. Published Pat. Appl. 2012/0172633; and Green Chem., 2014, 16, 695-707. Until recently, the proposed processes for manufacturing ethylene glycol from carbohydrates have suffered from extremely low selectivities to ethylene glycol. More recent proposals have focused on the use of two catalysts for the conversion of carbohydrates to ethylene glycol. One catalyst effects a retro-aldol reaction, and the second is used for hydrogenation. Thus, for instance, an aldohexose is converted to glycolaldehyde and erythrose, and the glycolaldehyde is in turn hydrogenated to ethylene glycol. Erythrose can undergo additional retro-aldol reaction to provide two more molecules of glycolaldehyde. Although the processes provide higher selectivity to ethylene glycol, a need still remains to provide a process that would be commercially competitive with conventional processes using ethylene as the feedstock.
Schreck, et al., in U.S. Published Pat. Appl. 2015/0329449, disclose improved, continuous processes for the conversion of carbohydrates to ethylene glycol and propylene glycol. They disclose using a reactor for the conversion of carbohydrates to the glycols which has a first zone comprising a retro-aldol catalyst and a second zone comprising a retro-aldol and reducing catalyst. Where the feed is an aldose, glycolaldehyde from the retro-aldol reaction is hydrogenated in the second zone of the reactor to ethylene glycol. They also disclose using ketose as the carbohydrate to produce propylene glycol.
Nevertheless, challenges still remain to further enhance the selectivity of the conversion of carbohydrates to glycols, especially to ethylene glycol. These challenges are not insignificant due to the myriad of reactions that can occur under the conditions required for the retro-aldol reaction and for the hydrogenation, including, but not limited to, hydrogenation of the hexose to hexitol and the formation of side products such as methane, methanol, ethanol, propanol, glycerin, 1,2-butanediol, threitol, and humins. Although some side products may be marketable, their recovery to meet merchant grade specification can be costly. Moreover, glycolaldehyde is highly reactive.